Additive manufacturing (also known as three dimensional printing) as practiced in industry has been, to date, mostly concerned with printing structural features. The main materials used are thermoplastics that offer form but not function.
There is great interest in the field of additive manufacturing to develop improved materials that can be used to easily print completely integrated functional objects with limited post-assembly. This would allow completely new designs in the manufacturing and consumption of everyday objects, particularly when they can be enabled with electrically conductive materials. The capability of printing conductive components within an object can provide the potential for embedded sensors and electronics.
Common techniques in additive manufacturing utilize the extrusion of molten polymer through a heated nozzle. This method is used in, for example, fused deposition modeling (FDM), where a filament is fed into a hot zone for continuous extrusion. The molten polymer can be deposited layer by layer onto a build plate in order to form 3D objects.
The functional part of the polymer (e.g. graphitic material for conductivity) must be present in the filament in an FDM process. A subset of FDM is Paste Extrusion (PE), which is used in the case of materials that are not rigid enough to be fed as a filament through the nozzle head. In both cases the composite polymer is typically made during melt processing in an extruder. For FDM, a filament is then produced and for PE the composite is used as is.
There are very few filament materials currently on the market which exhibit electrical conductivity, and those which are available have relatively low conductivities, which limits the range of potential applications. One example of a paper directed to the study of electrical percolation in such materials is Yao Sun et al., Modeling of the Electrical Percolation of Mixed Carbon Fillers in Polymer-Based Composites, Macromolecules 2009, 42, 459-463, which describes the use of multi-walled carbon nanotubes and either carbon black or graphite to lower percolation thresholds for polymer composites. This paper does not describe techniques for increasing conductivity substantially beyond the percolation threshold. Nor does it discuss the use of conductive polymers for additive manufacturing.
Achieving high loadings of conductive materials (e.g., graphitic materials) into a filament composite would enable high conductivity. However, these high loadings for typical additive manufacturing polymers (e.g. polycaprolactone, polyurethanes) would result in melt temperatures of over 250° C. or 300° C., which potentially renders the materials unsuitable for 3D printing because such high temperatures are generally not used in 3D printers. In addition even if one could attain these melt temperatures, polymer degradation would become an issue at such high temperatures.
Filament composites for FDM are often prepared using an extruder. Using an extruder to prepare a composite requires a minimum polymer viscosity for good mixing, dispersion and extrudability. The minimum viscosity may be in the tens of thousands of centipoise, for example. Thus, while lower viscosity polymer materials could be used to decrease melting temperatures of high carbon particle load materials, the minimum viscosity requirements of the extruder precludes the use of such low viscosity or low glass transition temperature (Tg) polymers in extruded FDM feed materials.
The process of emulsion aggregation (EA) is generally well known in some arts, such as for toner manufacturing. In a typical EA process, a latex is first aggregated by the judicious use of an aggregant that destabilizes the latex and allows controlled growth to a desired particle size. It is then stabilized and heated above the glass transition temperature (“Tg”) of the polymer to allow for polymer flow and coalescence of the resulting particles to from a larger homogenous polymer particle. In manufacturing toner processes, different materials (pigments, carbon particles such as carbon black, or waxes) are added during the EA process that can be incorporated in the final polymer particle. However graphitic materials such as Carbon Nanotubes (CNT) have not been used in the percentages (>5% by weight) required to enable conductive polymers (with conductivities typically greater than 1 S/cm).
There is therefore a need to have an efficient and inexpensive process for preparing new polymers that are heretofore been excluded from being used in FDM.